This invention relates to high-pressure sodium-vapor discharge lamps of the type having a resistive conducting element in the discharge space in order to assist discharge starting when cold.
Copending U.S. patent application Ser. No. 472,011 filed in the U.S. on May 22, 1974 and assigned to the common assignee of this invention, now abandoned, describes improved discharge starting in a high-pressure sodium-vapor discharge lamp by the introduction of a good electrical conductor in the discharge space. The presence of a large-diameter metallic rod or wire, i.e. a low-resistance conducting element, reduces the distance between the ends of the main electrodes across which the discharge is established. The effectiveness of the reduced distance enables the discharge to become established over a shorter distance between the electrodes when the lamp is cold. Once the lamp is started the discharge gradually occurs across the full distance between the electrodes corresponding to the steady-state condition. This low-resistance element described in the above identified application defines a first discharge space relative to either or both main electrodes to facilitate lamp starting across the first discharge space when the lamp is cold. When the lamp becomes heated the discharge then switches to the two main electrodes.
According to the arrangements described in the above identified patent application, an electrically resistive conductor is placed in the discharge space rather than a good electrical conductor, since the presence of a good electrical conductor lowers the physical resistance of the lamp in the steady-state condition, thereby reducing its life. This major disadvantage is the result of local overheating due to the presence of the conducting element in the discharge space. If, as described in the above identified application, a resistive element is employed in the form of a thin film on the wall of the burner, the improved starting of the lamp is offset by the deterioration of the deposited film. The deterioration of this film can produce cracks in the support surface (alumina or other material), resulting in destruction of the lamp.